This invention relates to monolithic ceramic capacitors and more particularly to such capacitors in which the capacitor electrodes are buried in a fine grained high dielectric-quality center layer that is sandwiched between two relatively low cost and coarse grained outer layers.
In the patent to Cipollini, U.S. Pat. No. 4,654,075, issued Mar. 31, 1987 and assigned to the same assignee as is the present invention, there is disclosed an emulsion-char method for making fine ceramic powder. The powder made by this method and other wet processes has smaller and more spherical particles and tends to have a much more narrow distribution of particle sizes than do powders made by the long conventional method of thermally reacting powdered oxides and oxide precursors of the wanted ceramic compound. Such fine and almost single-particle-size powders are sometimes described as mono-disperse powder. Furthermore, each particle of the fine powder made by the Cipollini method has the wanted chemical composition unlike in the conventionally produced powder particles and, therefore, the fine powder has a near ideal chemical homogeniety.
These characteristics of emulsion-char derived powders lead to lower cost capacitors or better capacitor dielectric properties, or both. For example, the finer particles are more reactive and can be fully densified at a lower sintering temperature, permitting the use of otherwise excluded lower melting, less expensive buried metal electrodes. The chemical homogeniety is carried along to the sintered capacitor dielectric ceramic leading to higher dielectric constant, K, and better control of K and the temperature coefficient of K as well as higher breakdown voltages and lower Q. But perhaps the most important advantage is the submicron particle size that permits the use of unusually thin active dielectric layers between electrodes. Such closely spaced electrodes, e.g. down to 0.2 mils (5 microns) in a ceramic of given K, are now made possible whereas 1 and 2 mil (25 and 50 microns) spacing has been the rule using the best of relatively coarse conventionally made powders.
However, conventionally made powders having relatively large powder particles are much less expensive to make than are the fine powders.
It is an object of this invention to provide a low cost compound-ceramic monolithic capacitor in which the capacitor electrodes are buried in a high-quality fine-grained center layer that is sandwiched between two relatively coarse-grained low cost outer layers to provide added thickness to facilitate handling and to contribute strength to the fine-grained center layer.